Field of the Invention
The present disclosure generally relates to imaging and, more particularly, to an image display apparatus and a method for controlling the image display apparatus.
Description of the Related Art
In recent years, there has been a user demand for image display apparatuses with high display performance, and a user demand for maintaining accuracy in gradation expression has been increased. To address such demands, as discussed in Japanese Patent Application Laid-Open No. 2012-14004, such an image display apparatus is becoming more widespread that includes a function of automatically measuring a characteristic value relating to display characteristics of a display unit (e.g., display colors and display luminance of the display unit) periodically with a measurement unit, such as a sensor, provided opposing the display unit and automatically calibrating the display characteristics in a case where the obtained characteristic value deviates from a predetermined value (i.e., calibration function).
In addition, in recent years, there has been a technique in which, in a liquid crystal display apparatus, a emission intensity of a backlight formed by a light source, such as a plurality of light emitting diodes (LEDs), is controlled in accordance with image data in each of a plurality of emission areas of the backlight corresponding to a plurality of areas constituting a screen. There is also a technique in which, in addition to controlling the emission intensity of the backlight in accordance with the image data in each of the emission areas, the image data is corrected in accordance with the emission intensity level. These techniques are referred to as local dimming control techniques, and the use of the local dimming control techniques makes it possible to improve a contrast in a displayed image and to suppress a misadjusted black level.
In a case where calibration control is carried out in an image display apparatus provided with the local dimming control technique (having a local dimming control function), the following problem may arise. The problem that may arise in a case where the calibration control is carried out in a liquid crystal display apparatus having the local dimming control function will be described with reference to FIGS. 9A and 9B. A liquid crystal display apparatus 1, which is provided with the local dimming control function, includes a sensor unit 2. The sensor unit 2 measures a characteristic value relating to display characteristics of the liquid crystal display apparatus 1 in an area 3 (hereinafter, referred to as a measurement target area 3) directly under the sensor unit 2, and a control unit (not illustrated) of the liquid crystal display apparatus 1 derives the display characteristics through calculation processing based on the obtained characteristic value.
More specifically, colorimetric patches of different densities are successively displayed on a display unit 4, and the sensor unit 2 measures chromaticity values or luminance values of the colorimetric patches as the characteristic value. The control unit then derives the display characteristics through the calculation processing based on the obtained characteristic value. The liquid crystal display apparatus 1 then automatically calibrates the display characteristics (i.e., carries out calibration). In addition, as illustrated in FIG. 9A, the image displayed on the liquid crystal display apparatus 1 is dark black in color in the measurement target area 3 and in an area around the measurement target area 3. In such a case, a backlight (not illustrated) included in the liquid crystal display apparatus 1 is controlled so that its emission intensity is reduced in the measurement target area 3 (i.e., local dimming control).
In other words, in a case where an image displayed on the measurement target area 3 is dark, the sensor unit 2 measures the characteristic value of the display unit 4 in a state in which the emission intensity is reduced in the measurement target area 3. However, depending on the performance of the sensor unit 2, when the emission intensity of the measurement target area 3 falls below a predetermined value, even if the colorimetric patches are displayed successively on the display unit 4, a difference in gradation among the colorimetric patches may not be able to be measured at a sufficient level due to insufficient luminance of the colorimetric patches. As a result, the accuracy of the calibration may be degraded. In addition, as illustrated in FIG. 9B, if the emission intensity of only the measurement target area 3, of which the emission intensity has been low, is raised excessively in order to obtain a more accurate measurement value, a so-called misadjusted black level may occur in which a black image is displayed in a whitish manner only in the measurement target area 3 due to the raised emission intensity. This causes the quality of the displayed image to be degraded, and the primary effect of the local dimming control function cannot be obtained. In particular, when a diagnostic image is displayed on a liquid crystal display apparatus for making a diagnosis in a medical site, the aforementioned misadjusted black level is extremely annoying to doctors.